What Risks Do Mixed K-type And J-type Thermocouples Bring To Hot Runners

On-site maintenance typically has the problem of random replacement and mixed use of K-type and J-type hot runner thermocouples, due to the similar appearance size. Many operators simply look at the thickness of the probe and the appearance of the wire, but ignore the crucial differences of alloy composition, thermoelectric potential curve, temperature resistance range and controller compensation parameters. The mixed use will bring irreversible hidden hazards to the precision of hot runner temperature management, the stability of moulding and the service life of the equipment, and will even produce large-area defective goods and the loss of the mould.

K type adopts chromel-alumel alloy, with wide temperature resistance range, strong high-temperature oxidation resistance, stable linearity, ideal for engineering plastics and flame retardant modified materials of 200-450°C. Type J is made of an iron-constantan alloy which is only suitable for medium and low temperatures below 350°C with poor high temperature stability and quick oxidation and drift. K-type is replaced by J-type, and the working environment at high temperature will cause the thermocouple to age rapidly, the measurement deviation to increase day by day, and the controller to misadjust heating power continually, thus resulting in long-term overheated decomposition or insufficient melt fluidity.

The thermoelectric potential curves of the two types are totally different. The temperature controller has the pre-fixed compensation settings K and J correspondingly. Mixed use will cause the cold junction compensation algorithm totally mismatched, the temperature display is severely distorted, and the heating regulation logic is disorder. It will look like the set temperature is rising while the real heating is dropping, which is the opposite control and leads to the runner carbonisation, the gate salivation and the weld line flaws in batches.

AND the insulating material and high temperature resistance rating of the two leads are different. Long-term mixed use in high temperature zone would cause accelerated ageing of J type insulation layer, leakage between positive and negative poles, short circuit failure and temperature runaway. But when the heating coil burns out, it can cause expensive mould disassembly and maintenance costs. The model of thermocouple interface of each mould should be clearly marked by standard management, and blind replacement by appearance should be prohibited and the original model should be closely matched, so as to minimise the quality and safety problems caused by mixed usage from the source.333